Steam turbines are machines that are used to generate mechanical (rotational motion) power from the pressure energy of steam. Steam turbines are comprised of a number of different size stages. Each stage has a set of moving and fixed blades. The moving blades are attached to the turbine's rotor, while the stationary blades are called a diaphragm. The diaphragm guides the steam to glide over the moving blades for producing rotary motion.
To maximize turbine efficiency, the steam is expanded as it flows through the turbine, generating work in the multiple stages of the turbine. These stages are characterized by how the energy is extracted from them and are known as either impulse or reaction turbines. In an impulse turbine, a stage is a set of moving blades behind the nozzle. In a reaction turbine, each row of blades is called a “stage”.
One problem which occurs in the operation of turbines is the occurrence of axial deflections and mechanical stresses in low pressure (“LP”) turbine casings. These axial deflections and mechanical stresses are conventionally controlled through the use of axially extending continuous internal ribs connecting all of the ledge rings in the turbine's LP casing. Ledge rings are the plates in an LP turbine casing which contain the steam seal faces with diaphragms. When axial defection is controlled through the use of continuous ribs connected to the inside surface of a turbine's LP casing between ledges, a lot of material and welding and manufacturing time is required, which can be expensive.
Controlling axial deflections and mechanical stresses in LP turbine casings can be further achieved by increasing the thickness of the ledge rings. However, after the thickness of the ledge rings has reached a certain thickness, it is no longer advantageous, cost wise to further increase the thickness of the ledge rings.